This invention relates to a method of converting organic wastes to valuable resources. The invention particularly relates to a method by which organic wastes in a solid, sludge or liquid form such as municipal wastes, night soil, sewage sludge and industrial wastes can be converted to valuable resources through a hydrothermal treatment.
Various methods are under development to provide for conversion of wastes into valuable resources through a hydrothermal treatment. According to one proposal, any solid matter in the waste to be treated is ground to particles no greater than several millimeters and a feed slurry is prepared that has a sufficiently high water content to be transported by means of a pump; the slurry is then subjected to a hydrothermal reaction to form a carbon slurry; since the carbon slurry is easily separable from water, it is dewatered and concentrated to have a heating value of at least 4,000-5,000 kcal/kg; the resulting concentrated carbon slurry can be used as a fuel.
The term "carbon slurry" as used herein means a mixture of water and the organic content which has been dehalogenated and deoxigenated in form of CO.sub.2 to carbides (called "char"), oils and water-soluble components. The "oils" are substances that are extractable with organic solvents such as dichloromethane. The term "aqueous phase" as used herein means the filtrate obtained by filtering the residue from the extraction with organic solvents. The term "solid phase" as used herein means the remainder that results from the removal of the oils and the aqueous phase. The term "hydrothermal treatment" means an operation in which the feed is pressurized to a level higher than the saturation vapor pressure at the reaction temperature and then heated to the reaction temperature to cause a chemical reaction. In the treatment of wastes such as municipal wastes, the reaction temperature is usually on the order of 300-350.degree. C. and the pressure is about 150 atmospheres.
This method, however, has the following problems. Depending on the waste to be treated, the feed slurry for the hydrothermal reaction may have low fluidity or high viscosity and the water content of the feed slurry must be increased to about 85-90% then, the water processing capacity of the facitity becomes or an unduly great energy load is required in subsequent heating and cooling operations.
In a waste treatment plant, the feed retaining facility usually requires a large enough capacity to absorb the input variations so that a uniform quality of wastes can be supplied to the facility at the subsequent stage. For instance, according to the Guidelines for the Structure of Waste Treatment Plants issued by the Welfare Ministry, it is required that a waste pit in incineration facility should have a capacity equivalent to at least two days of a projected maximal daily retention. As a matter of fact, the waste pit retains a large amount of wastes retains. In a hydrothermal treatment facility, the waste to be treated is desirably transformed to a feed slurry for the hydrothermal reaction and retained in the tank under mixing so as to provide uniformity in the slurry. In fact, however, due to the retention of a large amount of wastes, putrefaction of the slurry progresses to either elevate the COD in the aqueous phase or cause odor and hygienic problems.
After the hydrothermal reaction, the aqueous phase is separated from the carbon slurry and the concentration of the organic matter in the separated aqueous phase becomes significantly lower than that of the organic matter present in the aqueous phase of the feed slurry. Nevertheless, the separated aqueous phase still contains the organic matter at a concentration of 2-3.times.10.sup.4 ppm in the usual case and must be diluted with a large amount of water before it is subjected to a biological treatment. In the case of presence of heavy metals in the aqueous phase, a biological treatment is practically infeasible and a fuel must be employed to perform combustion, or an operation consisting of evaporation and oxidation treatments.
In order to remove the aqueous phase from the carbon slurry that has been subjected to the hydrothermal reaction, dewatering by filtration is performed in the subsequent concentrating step. However, the oils in the slurry will either adhere to the filter surface or clog the filter medium, requiring frequent cleaning of the filter. Therefore, continual operation of the hydrothermal reaction facility not only requires an auxiliary machine for filtration but also imposes a considerable operational load.
Thus, in spite of the substantial progress in the studies on the hydrothermal reaction perse, there have been many problems to solve before the technology for converting wastes into valuable resources in the form of a concentrated carbon slurry of high heating value by a hydrothermal reaction treatment can be implemented as a commercial process.